Robb I think the obvious reasons are that most antenna manufactures have problems when their designs may abuses their insulators, which are expensive to buy, costly to make and make fit, and are historically prone to failure when they don't use premium stuff or else embed the insulator inside a good support structure.
Thus most prefer to use "plumbers delight" construction in beams for sure and even among many verticals. I think most would forgo the dubious benefits of isolating the whole antenna considering the cost if nothing else, excepting maybe in particular with balanced antennas and now we find with the Sirio Gain Master there is a way around even that.
As a side note, BM recommended a company a while back that claims to sells Delrin. I bought two 26" x 1.125" pieces. I used one to insulate my A99 and had about 6" exposed on a 10' foot mast. After it had been up for about a month last summer, I took it down and noticed that it was bent. Only the heat of summer and the mild winds could have done that. You don't always get what you pay for, and I hate to say what I paid for that material.
I don't think I got Delrin.
If I've talked about insulating here on the Forum, I was probably speculating on the issue at advice of others. However, when I tried it...I installed it to the side instead of raising the antenna above the mast. So, that could have been the reason I got minimal results and did not see anything remarkable. In addition, I am beginning to understand that my location does some strange things at times, and it may be a good point to stop all my testing. It would surely be a lot easier to just start casting comments.
